1. Field of the Invention
The invention relates to a semiconductor packaging structure, and more particularly, to a structure of wafer-level chip scale package (CSP).
2. Description of the Related Art
As the technology of semiconductor fabrication grows more and more advanced, the relevant techniques have to be further developed to coordinate the requirements of the semiconductor devices. The fabrication process of a semiconductor device typically includes three stages. In the first stage, an epitaxy technique is used for the formation of a semiconductor substrate. Semiconductor devices such as metal-oxide semiconductor (MOS) and multilevel interconnection are fabricated on the substrate in the second stage. The third stage is the packaging process. It is now a leading trend for fabricating a device or an electronic product with a thin, light, and small dimension, that is, with a higher integration for semiconductor devices. In terms of packages, many techniques such as chip scale package, multi-chip module (MCM) have been developed to obtain a high integration. The development of the fabrication technique with a line width of 0.18 .mu.m has evoked a great interest and intensive research to further decrease the package volume. It is thus one of the very important package techniques to arrange more than one chips into a single package. In a multi-chip package, chips of processor, memory, including dynamic random access memory (DRAM) and flash memory, and logic circuit can be packed together in a single package to reduce the fabrication cost and the packaging volume. Furthermore, the signal transmission path is shortened to enhance the efficiency. The multi-chip IC packaging technology may also be applied to a multi-chip system with variable functions and operation frequencies, for example,
1. A system comprises memory chips, microprocessors, resistors, capacitors, and inductors.
2. A system comprises memory chips (DRAM), logic circuit chips, and memory chips (Flash memory),
3. A system comprises analog chips, logic circuit chips, memory chips (including DRAM, SRAM, Flash memory), resistor, capacitor, and inductor.
In FIG. 1, a conventional dual-chip module is shown. A substrate 10 comprising a copper pattern 12 is provided. By means of the formation of solder balls 14, the electrical connection to an external device or circuit is established. A very popular material of the substrate is polymide. A die 16 with a larger size is adhered onto the substrate 10 with an insulating layer 16 as a glue layer in between. An insulating layer 20 and a die 22 with a smaller size is then disposed on the insulating layer 16. Conductive wires 24 are formed to electrically connect the dies 18, 22 and the substrate 10. Using resin 26, the dies 18 and 22 and the substrate 10 are molded. The electrical connection between the whole package and a printed circuit board can be achieved by ball grid array (BGA) which use solder balls 14 to connect the terminals on the printed circuit board. The drawback of this conventional dual-chip module includes a large surface is occupied since dies are packaged on a same side of surface. The thickness of the package is as thick as 1.4 mm, therefore, the volume of the package is large, and the signal transmission path between chips is long.
To further shrink the volume of package, a face to face multi-chip package is disclosed in U.S. Pat. No. 5,331,235. This multi-chip package uses tape automatic bonding technique. The electrical connection between chips and printed circuit board is achieved by the installation of a lead frame or other carriers. The signal transmission path is lengthened. In addition, a large thickness and surface area are resulted by using the molding material (resin) of package. The applicability is reduced, and the heat dissipation is not effective. Moreover, this kind of package can not be applied to high frequency products.